1. Field of the Invention
The present invention relates to a power MOSFET and, more particularly, to a power MOSFET having a current sensing element of high accuracy.
2. Description of the Related Art
In the vertical type field effect transistor having a current sensing element according to the prior art, a small part (e.g., about 1/3,000) of active cells is separated from the source electrode of the field effect transistor to sense the current flowing through the field effect transistor as disclosed in U.S. P. No. 4,553,084.
FIG. 1 is a sectional view for explaining a vertical type field effect transistor according to the prior art. The field effect transistor has a common N.sup.- -type drain 2 and an N.sup.+ -type drain 1 formed thereunder. A plurality of bases 3 and a base 3' are formed with P-type in the N.sup.- drain 2. An active cell 20 and a current sensing cell 30 are formed by using the bases 3 and the base 3', respectively. N.sup.+ -type sources 4 and 4' are respectively formed in the bases 3 and 3'. Gate electrodes 5 are formed to overlay a portion between the adjacent sources 4, 4'. Source electrodes 7 for the active cell 20 are formed to contact the bases 3 and the sources 4, while source electrode 8 for the current sensing cell 30 is formed to contact the base 3' and the source 4'.
When sensing the current flowing through the active cell 20, the potential at the source electrode (or current sensing electrode) 8 of the current sensing cell 30 is different from that of the source electrode 7 of the active cell 20, since the source electrode 8 is connected to a load resistor (205 in FIG. 2) while the source electrode 7 is grounded. Furthermore, since the base region 3' forming the channel of the current sensing cell 30 is connected with the source electrode 8 of the current sensing cell 30, the gate-source bias of the current sensing cell 30 changes with the current flowing through the current sensing cell 30 the value of which is proportional to the current of the active cell 20. As a result, the sensed current is influenced by the current of the active cell 20 to be sensed. Thus, the sensing accuracy is not reliable.
FIG. 2 is a circuit diagram showing the case in which the current is sensed by the device in the prior art. In FIG. 2, reference numeral 200 designates a MOSFET, numeral 201, a current sensing MOSFET, numeral 202, a load resistor, numeral 203, a mirror terminal, numeral 204, a Kelvin terminal, numeral 205, a current sensing resistor, numeral 206, a comparator, numeral 207, a reference voltage, numeral 208, an output terminal, numeral 209, a gate drive circuit, numeral 210, a drain terminal, numeral 211, power source, numeral 212, a source terminal, numeral 213, the ground, and numerals 214 and 215, back gates (or bases).
The back gate 214 of the current sensing MOSFET 201 is electrically connected with the mirror terminal 203, while the back gate 215 of the MOSFET 200 is connected with the source terminal 212 and is connected to the mirror terminal 203 through the current sensing resistor 205. Therefore, the channel forming voltage of the current sensing MOSFET 201, i.e., the voltage difference between the gate and the back gate, becomes lower than that of the MOSFET 200 by the voltage drop across the current sensing resistance 205, so that the current sensing accuracy is lowered. The lower the gate voltage is, the more serious the drop in accuracy is.
Referring back to FIG. 1, when the potential of the current sensing electrode 8 is not fixed (e.g., when the current sensing terminal is not used), the potential of the base region 3' of the current sensing cell 30 floats, so that the extension of the deplection layer at the current sensing cell 30 is suppressed in comparison with the active cell 20. As a result, the breakdown voltage of the current sensing cell 30 is decreased.